I think it's because the media loves to scare people and they love to get a reaction. When they report on a person being attacked by a lab, there's not going to be a huge public outcry demanding that labs be banned from their town, because everybody knows somebody with a lab. But when they say a pit has attacked someone, they know that everyone is going to watch/read that story because it's just so controversial.
I remember when I was a little girl, a neighbor had a pomeranian (sp?). This dog would attack me each and every time I'd walk in front of her house. My dad repeatedly asked the mayor to do something about it, but everyone always just said "Oh, it's a pom! What damage can it do?!" My legs were covered in bites. Then when I was twelve or so, a big black stray named Joe decided I was his human. He followed me around and was extremely gentle. One day I was walking by the neighbor's house, though, and the pom came out and attacked me. Joe grabbed the pom by the neck and threw it back. Unfortunately, the pom died from the injury. The entire town rallied around this neighbor and a week later we found Joe a couple miles away, shot. It turned out the mayor himself had shot Joe. The local newspaper ran an article on the incident. It didn't say that the pom had attacked me and Joe was protecting me, it said that an innocent pom had been MAULED to death by a chow-mix. They knew that that story would get more papers sold than the truth.
EDIT: They completely made up that Joe was a chow mix. We didn't know what the he** he was, he was a mutt. He didn't have the black spots on his tongue, and to me he looked more like a lab, even now when I look at pictures of him.